There are only 12 forms that fit this situation--you can e-file, but must also mail the paper form. The most common are a 1098-C (donation of a motor vehicle), form 8283 (certain non-cash charity donations) and forms 8332 (given to you by the custodial parent of your child, allowing you to claim the child as a dependent even though they lived with you less than half the year.)
All 12 forms are listed on form 8453, the "transmittal" form, which is basically a cover sheet that you mail with whatever other form you must mail, so the IRS knows what to do with your form when they receive it. Here is a copy of the blank form 8453, listing all 12 forms that could be attached.
https://www.irs.gov/pub/irs-pdf/f8453.pdf
If you view or print the PDF of your tax return, or the PDF for "all forms and worksheets", it should include a copy of the transmittal form 8453 already filled out and with a checkbox indicating what form it should be sent with. Most of those other forms should be part of your tax return PDF. Form 8332 is given to you by the other parent of your children and form 1098-C is given to you by the charity to which you donated a vehicle. If you need to mail form 8283 for certain non-cash donations over $5000, you may need to get the form signed by an appraiser and an office of the charity.
Using most PDF software, you can selectively print form 8453 plus whatever other form is checked on the front. If you are mailing a form 8332 as a result of a custody order, you mail the signed original but keep a copy for yourself just in case.
